Called from SubsystemManager before reloading all XR SDK Provider packaged subsystems.
When the Editor starts or when packages are installed or removed, the SubsystemManager searches the packages and loads all XR SDK packages that it finds. Handling this event allows the user to do work they may need prior to the subsystem manager cleaning up any current subsystem descriptors.
Additional resources: SubsystemManager, SubsystemDescriptor.
*Note:* This is deprecated, use beforeReloadSubsystems instead.